The fiber reinforcement of rubber technology described in this disclosure optimizes the properties of the rubber, and thus is useful in many industrial or commercial applications. The application of the technology to optimize the properties of rubber used in PDM stators will be used in this disclosure to illustrate one such application.
This disclosure describes a range of optimized fiber-reinforced rubber compositions, and methods of making them, for use in the stator injection process. During the rubber injection process to make stators, the rubber is injected though a mold that requires the rubber to flow through a geometry with a very high length to cross section ratio. Typical stator tube geometries may have lengths of 120″ to 300″ for tube diameters of 4.75′ and larger. Stator tubes in the 2″ to 4″ diameter range have typical lengths of 60″ to 150″ and stator tubes in the 1.5″ to 2″ have typical lengths of 50″ to 100″. As a result of the injection flow process to achieve these geometries, significant grain direction at the rubber molecular level is established in the lobes of the stator. The establishment of a grain in the flow direction is unavoidable, creating undesirable anisotropy in the rubber when cured.
Rubber anisotropy in the stator causes the material properties of the final rubber product to be different in the cylindrically transverse cross-section direction of the stator than in the cylindrical longitudinal direction. In fact, rubber flow during injection is more accurately in a helical pathway flowing in a generally longitudinal direction. Thus the rubber chain molecule grain follows a helical pathway, although performance metrics of the stator look more closely in the cylindrical longitudinal direction and the cylindrical transverse cross-section.
Persons of ordinary skill in this art will understand that, consistent with applicable standards such as ASTM D412, terms such as “Young's Modulus”, “Modulus of elasticity”, “tensile Modulus”, or just “Modulus” (as used in this disclosure) are interchangeable to describe a parameter representing the general propensity of a material to deform (elongate) under a tensile stress load. The value of Modulus for a particular material is generally measured in Pascals, and quantifies the material's propensity to deform under tensile load. The value of Modulus thus predicts an elongation in the material (or a. “strain” in the material) for a given tensile stress load. Conversely, the value of Modulus predicts the tensile stress required to be applied to the material to achieve a certain elongation (or “strain”). Thus, by way of example and again consistent with ASTM D412, the term “25% tensile Modulus” or “25% Modulus” as used in this disclosure refers to the tensile stress applied to a. material (or seen in a material) at 25% elongation, “50% tensile Modulus” or “50 % Modulus” refers to the tensile stress applied or seen at 50% elongation, “100% tensile Modulus” or “100% Modulus” at 100% elongation, and so on. Modulus is one important material performance property of rubber in PDM stators. Modulus is also a somewhat reliable indicator of other desirable material properties, in that higher Modulus will normally indicate higher tensile strength and crack resistance. Without some sort of reinforcement, the rubber anisotropy inevitably caused by injection molding in stator manufacturing causes the cured rubber to exhibit lower Modulus in the cylindrical transverse cross-section direction (“against the grain”) versus in the cylindrical longitudinal direction (“with the grain”). Low Modulus in the transverse direction leads to premature breakdown and “chunking” of the rubber under cyclic operational loads in a PDM.
Elongate fibers introduced into the rubber strengthen the rubber composite, and improve material properties such as crack resistance. When added to rubber, small amounts of fiber can significantly improve the life of components by acting to distribute stress across the component more effectively. This is particularly effective as the component weakens during cyclic loading. Fibers distribute and dissipate energy at the crack tip of any flaw initiation site, thereby slowing the crack initiation and propagation stage of fatigue failures.
Unfortunately, however, elongate fibers within a rubber composite are susceptible to the same grain alignment during manufacture as the underlying rubber chain molecules. Thus conventional fiber-reinforced rubber composites do little to address loss in transverse Modulus, for example, due to the underlying rubber molecule chain anisotropy. The reality is that when elongate fibers are added to the rubber composition, the fibers also tend to align substantially with the grain, i.e., in the flow direction of the helical path of the lobe geometry. The most significant changes in material physical properties enabled by the fibers will be aligned with this helical path and substantially along the length of the stator. In the transverse cross-sectional direction, the material properties will tend to change less. In order to enhance transverse material properties such as transverse Modulus, therefore, it becomes desirable to load the fiber content of the rubber as high as possible, and/or to use high strength fibers as much as possible. However, high fiber load, and/or use of high strength fibers may cause other performance issues with the rubber composition, both in chemistry and in material properties. In particular, high fiber load and/or use of high strength fibers is known to reduce flexibility and cracking resistance in some applications, especially at lower temperatures. There has been a longfelt but unsolved need in the PDM stator art for rubber composite products that carry a high fiber load and/or use high strength fibers, and that have also maintained serviceable chemistry or material properties in other aspects.
U.S. Pat. No. 6,358,171 to Whitfield discloses fiber loading of a rubber composite in tension belt applications (such as automotive timing belts). In column 3, line 65 through column 4, line 9, Whitfield posits that the dispersed fibers inhibit crack propagation and growth in the belt rubber during operational loads, thereby improving performance of the belt at both high and low temperatures. Whitfield further discloses that the fibers increase the shear strength of the teeth and thus provide a higher load-carrying capability than a similar belt made without fiber reinforcement.
While instructive on the operational benefits of fiber-reinforced rubber composites generally, Whitfield does not address the anisotropy problem in the PDM stator art identified above, namely achievement of serviceably high Modulus in the transverse cross-section direction (“against the grain”) when the manufacturing process necessarily creates substantial fiber alignment in the longitudinal direction (“with the grain”). As can be seen from the Figures in Whitfield, the fibers are aligned in the direction of travel of the belt. Because the belt is retained by pulleys in operation, the belt undergoes comparatively little load in the transverse direction (“against the grain”).
The rubber composition disclosed by Whitfield nonetheless forms a serviceable starting point from which to develop a new rubber composition, as disclosed in this application. The modified rubber composition will address the problems in the PDM stator art described above.
U.S. Published Patent Application 2015/0022051 to Meng et al. (“Meng”) discloses a fiber-reinforced rubber composite material for use in PDM stators. In paragraph 0008. Meng identities reasons why the prior art has had difficulty deploying such fiber-reinforced rubber composites in injection molding manufacturing process (such as are generally used in PDM stator manufacturing), and further identifies poor fiber dispersion throughout the composite matrix as a primary culprit. Meng improves dispersion via use of a solid “fiber dispersion compound”, such as amorphous silicon dioxide, admixed with the fibers into the rubber. Although Meng confines its disclosed embodiments to use of such a solid fiber dispersion compound, Meng defines “fiber dispersion compound” to include solid agents, liquid agents or a combination of both. Meng discloses use of a fairly wide variety of fibers (see paragraphs 0039-40), and in particular the use of high-strength aramid fibers such as KEVLAR® fibers, in which the chain molecules in the fibers are highly oriented along the fiber axis so the strength of the chemical bond can be exploited.
While Meng's use of a solid dispersion agent may improve dispersion, and thus improve the material properties of the fiber-reinforced rubber compound generally, Meng does not address the problem of anisotropy in PDM stator manufacturing. As a result, Meng estimates a fiber loading for the stator that is too low for optimum performance in the transverse cross-section direction (“against the grain”). As disclosed in paragraph 0062 of Meng, low fiber loading is preferred in Meng's composites in order to render minimal impact on properties other than Modulus.
U.S. Pat. No. 8,944,789 to Butuc et al. (“Butuc”) discloses reinforcing a rubber composite with a variety of “reinforcing agents” including fiber. Disclosed embodiments in Butuc use aramid fibers such as KEVLAR® fibers. Butuc also discloses use of a “dispersing substance” that is a carrier for the reinforcing agent. Butuc confines its disclosure to solid dispersing substances that include clay, glass, fumed silica, silicon dioxide, diamond and combinations thereof.
Butuc farther discloses use of magnetically-responsive particles to be included with the reinforcing agents and dispersion substances. In FIG. 4C and associated disclosure, Butuc activates a magnetic source through the longitudinal center of the stator during curing of the rubber composite, with the goal of causing the magnetically-responsive particles to align the reinforcement fibers towards the source. As a result, the “grain” in such stators is substantially uniformly in the transverse cross-section direction.
While such magnetically aligned fibers may cause the stator to have improved properties (such as Modulus) in the transverse cross-section direction, Butuc's magnetic method leaves several drawbacks that do not address or remediate the anisotropy problem identified above in this application, at least in any practical way. First, the magnetic alignment method of Butuc simply shifts the anisotropy problem into a different plane. After magnetic processing, there is operational weakness in the stator in the longitudinal direction, which is now “against the grain”. Butuc acknowledges as much in column 13, lines 3-14 of its disclosure. Second, the magnetic processing creates an additional manufacturing step which will add to the manufacturing cost of the stator. Third, there is no disclosure in Butuc regarding what effect, if any, the magnetically-responsive particles may have on the material properties of the finished stator. Finally, there is no validation in Butuc (e.g. via disclosed experimentation or examples) that the magnetic alignment process actually produces the transversely-aligned fibers as suggested.
There is therefore a need in the art for a rubber composition for use in, for example, PDM stators, that is engineered to address anisotropy problems caused by the inevitable fiber alignment seen in the longitudinal helical direction when injection molding is used in manufacturing. Advantageously the new rubber composition will use a simple manufacturing solution such as high fiber loading in order to generate serviceable material properties such as high Modulus in the transverse cross-section direction (“against the grain”).
While serviceable and highly advantageous in its own right to address anisotropy problems, high fiber loading may enable yet further benefits in some applications when short aramid fibers are used in the high fiber loading.
Generally speaking, highly fibrillated aramid fibers are advantageous in applications where high fiber loading is used to address anisotropy. Highly fibrillated fibers provide increased surface branching, and thus higher fiber surface area. The higher the cumulative fiber surface area, the more fiber reinforcing that becomes available to the mix.
However, more highly fibrillated fibers tend to interlock and, as a result, form fiber clumps and cause more problems with even fiber dispersion and distribution throughout the mix. It is known to extend mixing times to improve fiber dispersion, but extended mix times are also known to increase production cost, add mechanical stress to the finished elastomer, and increase the heat buildup in the batch during mixing. The increased mechanical stress and/or heat buildup leads to adverse effects on the compound during manufacture, such as molecular cleavage and premature scorching.
Another method used to improve fiber dispersion is disclosed in U.S. Pat. No. 8,944,789 to Butuc, as described above. Butuc teaches use of a dispersing substance such as clay, glass, fumed silica, silicon dioxide, or diamond. A disadvantage of using such dispersing substances is that they introduce an extraneous component to the compound that may adversely affect physical properties. For instance, finned silica and silicon dioxide are known to absorb water and thus increase the tendency of the finished rubber compounds to swell when exposed to water.
Still another method used to improve fiber dispersion is to pre-disperse the fiber in liquid dispersion agents (such those as disclosed in U.S. Published Patent Application 2015/0022051 to Meng et al.). Other methods are known to pre-disperse fiber in a low molecular weight oil and/or elastomer. Such pre-dispersion agents are conventionally mixed as a masterbatch. The masterbatch is then added at selected points through the manufacturing steps of the mix. Attempts to improved distribution of fibers in the mix via pre-dispersion agents thus have the disadvantage of reducing manufacturing economy, since additional masterbatch steps are required. Further, adding the pre-dispersion agent to the rubber compound may adversely affect in-service properties of the final compound, such as retention of mechanical properties at elevated temperatures.
There is therefore also a need in the art for a technique to improve dispersion of highly fibrillated aramid fibers in rubber compounds with high fiber loading. Advantageously, such a technique will not rely on known methods to promote dispersion, such as use of solid or liquid dispersion agents in the fiber/rubber mix.